Recent years, the performances of electronic parts in the electronics fields have been remarkably enhanced. This contributes to the enhancement of the information-processing speed of information processing units such as large scale computers, mobile communication terminals, personal computers, and so forth, the reduction in size thereof, and the realization of the multi-functions thereof. As one of these electronic parts, a multi-chip module (MCM) is exemplified in which a plurality of semiconductor devices such as VLSI, ULSI, and so forth are mounted onto ceramic substrates. In such a module, a ceramic multilayer substrate having wiring-conductors three-dimensionally arranged is often used.
The ceramic multilayer substrate is produced by firing a green multilayer mother substrate which are formed by laminating a plurality of ceramic green sheets. However, when the green multilayer mother substrate is fired by an ordinary technique, the multilayer mother substrate is shrunk in the main surface direction and in the thickness direction. Especially, this causes a dimensional error of about 0.4 to 0.6% in the main surface direction. Then, in some cases, the positional accuracies of external conductors are deteriorated, and internal conductors are deformed, distorted, or disconnected.
Accordingly, Japanese Unexamined Patent Application Publication No. 4-243978 proposes a method of producing a ceramic multilayer substrate as described below.
First, glass ceramic power capable of being sintered at a temperature of up to 1000° C., and alumina powder which can not be sintered at the sintering temperature of the glass ceramic powder are prepared. Subsequently, ceramic green sheets containing glass ceramic powder are laminated. Shrink-suppressing layers containing alumina powder are arranged so as to sandwich the green multilayer substrate to produce a green composite laminate.
Subsequently, the above-described composite laminate is fired under the sintering conditions for the glass ceramic powder and the alumina powder contained in the shrink-suppressing layers is not substantially sintered. Thus, the shrink suppressing layers are not substantially shrunk. According to this action, the shrink-suppressing layers constrain the green multilayer substrate. The multilayer substrate is shrunk only in the thickness direction, and the shrinkage in the main surface direction is suppressed. Thereafter, the shrink-suppressing layers are removed by a proper means to obtain the ceramic multilayer substrate.
The ceramic multilayer substrate obtained by the above-described so-called non-shrink process has a high dimensional precision in the major plane direction, and has a high reliability, exhibiting less warp and distortion.
Moreover, to efficiently produce the ceramic multilayer substrate, the following method is adopted. That is, a plurality of ceramic multilayer substrates are combined to form a multilayer mother substrate. The multilayer mother substrate is divided along predetermined dividing lines, so that a plurality of the ceramic multilayer substrates are obtained at once.
To divide the multilayer mother substrate as described above, grooves are provided along the predetermined diving lines on the major surface. Thereby, the multilayer mother substrate can be easily divided.
The grooves are ordinarily formed on the major surface of the green multilayer mother substrate by means of a cutter knife, a mould, or the like. In the case in which the ceramic multilayer substrate is produced using the shrink suppressing layers as described above, the grooves are formed as described in Japanese Unexamined Patent Application Publication No. 7-99263 and shown in FIG. 12.
FIG. 12 shows a cross-sectional view showing a part of a green composite laminate 101 comprising a green multilayer mother substrate 102, and a first shrink-suppressing layer 103 and a second shrink-suppressing layer 104 arranged so as to sandwich the substrate 102. In FIG. 12, a wiring conductor provided for the multilayer mother substrate 102 is omitted, and the size in the thickness direction is magnified.
In FIG. 12, the green multilayer mother substrate 102 comprises a plurality of ceramic green sheets 107 containing glass ceramic powder. The first shrink-suppressing layer 103 and the second shrink-suppressing layer 104 are formed by laminating a predetermined number of green sheets 108 containing sintering-difficult powder incapable of being sintered at the sintering temperature of the above-described glass ceramic powder, such as alumina powder or the like.
The green composite laminate 101 is produced as follows.
First, the ceramic green sheets containing glass ceramic powder are laminated, and pressed in the lamination direction to obtain the green multilayer mother substrate 102 containing a plurality of the ceramic green sheets 107. Thereafter, grooves 106 are formed on one main surface of the green multilayer mother substrate 102.
Subsequently, the green sheets 108 containing the alumina powder are laminated so as to sandwich the green multilayer mother substrate 102, whereby the first shrink-suppressing layer 103 and the second shrink-suppressing layer 104 are formed. Thus, the green composite laminate 101 is obtained. Thereafter, the green composite laminate 101 is pressed again, and fired at a temperature at which the glass ceramic powder contained in the ceramic green sheets 107 can be sintered.
Thereafter, the green first shrink-suppressing layer 103 and the green second shrink-suppressing layer 104 which are not sintered substantially in the above-described firing process are removed. Thus, the fired multilayer mother substrate 102 having the grooves 106 is obtained. The fired multilayer mother substrate 102 is divided along the grooves 106, and the individual ceramic multilayer substrates are obtained.
As described above, when the grooves 106 are formed on the green multilayer mother substrate 102, a plurality of the laminated ceramic green sheets need to be sufficiently pressed beforehand, so that the respective ceramic green sheets 107 are not shifted when the grooves are formed.
However, when the green composite laminate 101 is pressed again, it is difficult to obtain sufficient adhesion strengths between the green multilayer mother substrate 102 which have been already pressed and the first shrink-suppressing layer 103 and the second shrink-suppressing layer 104 which have not been pressed yet. As a result, the constraining forces of the first shrink-suppressing layer 103 and the second shrink-suppressing layer 104 for the ceramic green sheets 107 are small. Thus causes problems in that the shrinkage of the ceramic green sheets 107, that is, the shrinkage in the plane direction of the green multilayer mother substrate 102 can not be sufficiently prevented.
Moreover, in view constituting the multilayer mother substrate and the green sheets 108 constituting the respective shrink-suppressing layers, a process of forming the grooves 106, which is different from the laminating process, is carried out. Thus, problematically, the production efficiency is reduced.
In view of the foregoing, the present invention has been devised. An object of the present invention is to provide a method of producing a ceramic multilayer substrate by which a ceramic multilayer substrate can be produced at a high production efficiency, and a green composite laminate obtained in the production process.